The subject matter of the present invention relates generally to panel presses with movable platens to produce pressed panels of wood particles or other material with a predetermined thickness by controlling the press openings, and in particular to such a press in which the movable platens are individually controlled by position-sensor transducers which produce position signals for such platens that are used to actuate control cylinders for adjustment of each platen. Preferably, the position-sensor transducers are in the form of sonic waveguides which are mounted on a fixed upper platen and positioned adjacent permanent magnets which are fixed to the movable platens. In one embodiment the sonic waveguide transducers indicate the position of the movable platens by sensing sonic waves produced within a waveguide tube of magneto-strictive material when a moving electromagnetic field produced by an input pulse current flow in a conductor interacts with the stationary fields of the magnets so that the position of each movable platen may be determined by measuring the time period between an input pulse applied to the transducer and an output pulse of the transducer corresponding to the sonic wave.
It has previously been proposed in U.S. Pat. No. 4,222,724 to Van Hullen, issued Sep. 16, 1980, to provide a platen press having movable platens for pressing panels such as chipboards by controlling the press openings using platen adjustment cylinders. Each of such cylinders is connected to a different movable platen and such cylinders are simultaneously actuated for simultaneously closing the movable platens. Groups of cylinders are provided with each group mounted on a common support assembly which is pivotally attached to the frame adjacent the fixed upper platen. This multiple platen press has the advantage that it eliminates the need for previously used spacer strips positioned between adjacent platens to limit the platen openings to a predetermined spacing for controlling the thickness of the panels produced thereby. However, while this multiple platen press is less complicated than previous presses since it eliminates the need for spacer strips, it is not as accurate in producing panels of a predetermined thickness. Thus, the Van Hullen patent admits that the chipboard panels produced by such multiple platen press have small deviations from the desired thickness of the panels. This problem is solved by the multiple platen press of the present invention which employs position-sensor transducers such as sonic waveguide transducers with permanent magnets fixedly attached to the movable platens to produce position signals which accurately represent the positions of the movable platens during the operation of the press.
Sonic waveguide transducers have previously been employed for measuring liquid levels and other non-analogous purposes as described in U.S. Pat. No. 4,952,873 to Tellerman, issued Aug. 28, 1990. The Tellerman patent describes the use of sonic waveguides and a movable magnet to measure the level of a liquid in a tank by providing the magnet on a float which moves with the liquid level. However, there is no suggestion in this patent of employing such a sonic waveguide transducer to sense the position of movable platens in a multiple platen press in the manner of the present invention. The Tellerman patent also refers to his earlier patents on sonic waveguide transducers, including U.S. Pat. No. 3,898,555 to Tellerman, issued Aug. 5, 1975, U.S. Pat. No. 4,298,861 to Tellerman, issued Nov. 3, 1981, U.S. Pat. No. 4,721,902 to Tellerman, issued Jan. 26, 1988, and U.S. Pat. No. 4,726,226 to Tellerman, issued Feb. 23, 1988. However, these patents all show other non-analogous uses for the transducer. For example, Tellerman U.S. Pat. No. 4,298,861 patent relates to the use of a sonic waveguide transducer for encoding magnetic keys on a keyboard. Thus, in none of these prior art patents is the sonic waveguide transducer employed to sense the position of movable platens in a press for producing panels of pressed wood or other material of a predetermined thickness in the manner of the present invention.
U.S. Pat. No. 4,499,821 to Clouston, issued Feb. 19, 1985, relates to a system for immobilizing a plywood press platen during the press cycle. This patent describes a press including a system for immobilizing the hydraulic ram cylinder and thrust platen of a plywood press moved by such ram cylinder during the panel pressing cycle to avoid excessive panel compression. This is achieved by a valve assembly, a timing means and feedback means for sensing thrust platen movement and mechanically actuating the valve assembly. The Clouston system is not employed to control the movable intermediate platens in a multiple platen press as does the present invention which instead employs platen position-sensing transducers, such as a sonic waveguide and associated magnets, for sensing the positions of the movable platens and actuating control cylinders to more accurately control the final spacing between the platens for determining the thicknesses of the panels.